A spray arm 1a of a conventional dishwasher is, as shown in FIGS. 1 and 2, rotatably mounted at the center of a feed water pipe 13 which is installed on the bottom 20 of a dish washer. The spray arm 1a has washing nozzles 10 on upper part thereof for spraying washing water toward dishes, and rotation nozzles 11 for generating rotation force by the reaction of washing water sprayed therefrom.
When the feed water is, pressurized by a pump(not shown) and fed through the feed water pipe 13, sprayed from the rotation nozzles 11, the spray arm 1a is rotated by the reaction of the feed water spray from the rotation nozzles 11 only in one direction while washing the dishes by the spraying water from the washing nozzles 10.
In another conventional spray arm 16 as shown in FIGS. 3 and 4, when the feed water, pressurized by a pump(not shown), is fed inside of the spray arm 1b, ball 6a in the spray arm 1b, moved by water pressure, close rotation nozzles 4a and 5a alternatively changing the direction of spray resulting in a to change of the direction of rotation of the spray arm.
However, the conventional spray arm 1a as shown in FIGS. 1 and 2 described above has low washing efficiency due to the existence of a dead zone of spray developed by the one direction rotation of the spray arm 1a forced by water sprayed from the rotation nozzle 11 provided for producing a rotation force.
On the other hand, the conventional spray arm 1b in FIGS. 3 and 4, which has ball 6a in the spray arm 1b, resting in the neutral position before starting, repeating the action of closing either one of the rotation nozzles on feeding of water and returning to the neutral position on stop of the feed of water, also has low washing efficiency because the spray arm 1b can hardly be expected to rotate reversing periodically, as the probability of closing either one of the rotation nozzles 4a and 5a by the ball 6a is half and half resulting in the closing of the rotation nozzles being irregular, contrary to the required sequential closing of the rotation nozzles 4a and 5a by the ball 6a for the periodic reversing of the spray arm 1b.